1. Field of the Invention
The present invention relates to a speaker apparatus for playing back sound by applying vibration to an acoustic vibration plate by an actuator, such as a magnetostrictive actuator, and a method for driving the speaker apparatus.
2. Description of the Related Art
A speaker apparatus for playing back sound by applying vibration to an acoustic vibration plate by an actuator, such as a magnetostrictive actuator, has been developed.
As shown in FIG. 18, in one of the speaker apparatuses of such type, a driving rod 95 of a magnetostrictive actuator 90 is contacted to a plate-shaped acoustic vibration plate 81 to apply vibration to the acoustic vibration plate 81 in thickness direction thereof, that is, a direction perpendicular to a plate surface.
In another one of the speaker apparatuses of such type, as disclosed in Japanese Unexamined Patent Application Publication No. 2007-166027 and shown in FIG. 19, for example, a cylindrical acoustic vibration plate 85 with both ends open is supported vertically, and a plurality of magnetostrictive actuators 90 are disposed on the lower end side of the acoustic vibration plate 85 such that the driving rods 95 of the magnetostrictive actuators 90 are contacted to a lower end surface 86 of the acoustic vibration plate 85 to apply vibration to the acoustic vibration plate 85 in a direction perpendicular to the lower end surface 86, i.e., the plate surface direction.
In a speaker apparatus of the type shown in FIG. 19, although the lower end surface 86 of the acoustic vibration plate 85 is excited by a longitudinal wave, propagation of a vibration elastic wave in the plate surface direction of the acoustic vibration plate 85 mixes the longitudinal wave and a transverse wave, whereby a sound wave is radiated in directions perpendicular to the plate surface of the acoustic vibration plate 85 by the transverse wave. Thus, a spatial sound field is obtained.
A magnetostrictive actuator is an actuator using a magnetostrictive element which is deformable upon application of an external magnetic field. The amount of deformation of some magnetostrictive elements these days are nearly 1000 times the typical magnetostrictive elements (super-magnetostrictive elements), and magnetostrictive elements produces large stress when they are deformed. Thus, even a small magnetostrictive actuator can sound an acoustic vibration plate at relatively large sound volume, and it can sound even a hard acoustic vibration plate, such as an iron plate.
In addition, magnetostrictive actuators have excellent response speed. The response speed of a solitary magnetostrictive element is on the order of nanosecond.